Recently, in view of energy saving, resource saving and reduction of environmental burdens, hot melt adhesives have been widely used. Block copolymers (SBS, SIS etc.) composed of a monoalkenyl aromatic compound and a conjugated diene compound are widely used as base polymers of such hot melt adhesives. However, in such adhesive compositions obtained by using these block copolymers, the balance of the holding power, tack properties and processability is insufficient, and their improvement has been desired. JP-A-61-278578 discloses, for example, an adhesive composition comprising a triblock copolymer and diblock copolymer as a method for improving the above properties. In addition, JP-A-63-248817 discloses an adhesive composition comprising a block copolymer obtained by coupling using a specific bifunctional coupling agent (specific dihalogen compound).    Patent Document 1: JP-A-61-278578    Patent Document 2: JP-A-63-248817
However, in recent years, properties required for hot melt adhesives have become more and more advanced and complicated. For example, adhesives used for sanitary goods such as disposable diapers and sanitary napkins are expected to have a wide range of properties such as transparency, heat discoloration resistance, deodorization and low viscosity in view of the appearance, sanitation and processability. Moreover, in response to the demand for improvement in the technique of coating of adhesives and the corresponding demand for increasing the productivity of final products, adhesive compositions need to have a low viscosity. Also, for retaining the strength of the object to be bonded, hot melt adhesives having excellent coatability at lower temperatures are in demand.
However, SBS, for example, generally has poor heat resistance because it contains a vinyl bond in the conjugated diene chain, and when the viscosity of the adhesive is reduced at high temperature so as to improve the processability, for example, heat discoloration, gelation or thermal decomposition may occur and impair the appearance of the adhesive composition, and adhesive performances may be deteriorated. On the other hand, when an adhesive composition is processed at low temperature to suppress such heat discoloration and heat degradation, processing is difficult due to high melt viscosity. Further, when taking processability into account, a method in which a large amount of a softener is added to improve the processability or a method in which the amount of addition of the block copolymer is reduced may be used. However, in these methods, the balance of adhesive performances may be deteriorated or bleeding of the softener may occur, often resulting in degradation of the appearance and sanitary properties of the adhesive composition. Moreover, although simple reduction of the molecular weight of SBS improves the heat discoloration or processability of the resulting adhesive, this is not preferred because adhesive performances are significantly deteriorated.